1. Field of the Invention
The present invention generally relates to a method of forming solder bumps and a method of forming preformed solder bumps, and particularly relates to a method of forming solder bumps and a method of forming preformed solder bumps by heat fusion of a printed solder paste.
2. Description of the Related Art
In the related art, several methods are known for forming solder bumps on a semiconductor device and a circuit board. Those methods may be a solder plating method, a method using solder paste and a method using solder balls. A method of forming solder bumps using solder paste is widely employed since solder bumps may be formed by a screen printing process which requires simple equipment and a low cost.
Along with the recent miniaturization of circuit elements, solder bumps have become increasingly small and their pitch has become increasingly fine. Also, in order to mount the circuit elements accurately, solder bumps are required to be formed with high reliability.
Therefore, a method of forming solder bumps is required which can form solder bumps easily with high reliability.
Referring to FIGS. 1 to 4, an example of a method of forming solder bumps according to the related art will be described. In the following, the related art will be described in detail by way of an example of a method of forming solder bumps 102 on a semiconductor chip 100.
FIG. 1 shows the semiconductor chip 100. Here, the solder bumps 102 are not yet formed on the semiconductor chip 100. The semiconductor chip 100 is provided with a plurality of electrodes 104 on a mounting side of a supporting board 106. The term mounting side is to mean a surface of the supporting board which is to be mounted on a mounting board. In order to form the solder bumps 102 on the semiconductor chip 100, as shown in FIG. 2, first, a mask 108 is formed on the surface of the supporting board 106 provided with the electrodes 104. The mask 108 is provided such that mask openings 110 are formed at positions where the electrodes 104 are formed (i.e., where the solder bumps 102 will be formed in the following process).
After the mask 108 has been provided on the supporting board 106, solder paste 112 is provided in the mask openings 110 formed in the mask 108, as shown in FIG. 3. A screen printing process may be utilized as a method of providing the solder paste 112 in the mask openings 110. The solder paste 112 is provided on the electrodes 104 by filling the mask openings 110 with the solder paste 112.
Then, after removing the mask 108 from the supporting board 106, the semiconductor chip 100 is subjected to a heating process in a reflow oven. In this process, an organic paste contained in the solder paste 112 evaporates and solder particles are fused, so that the solder bumps 102 are formed. Thus, as shown in FIG. 4, the semiconductor chip 100 having the solder bumps 102 is formed.
The semiconductor chip 100 provided with the solder bumps 102 is mounted on the mounting board by flip-chip mounting. In order to achieve a high reliability mounting, the solder bumps 102 are required to have a certain height. In other words, if the height of the solder bumps 102 is too small, the solder bumps 102 cannot eliminate an irregularity caused by, for example, a roughness of the surface of the mounting board or a deformation of the mounting board due to thermal expansion. In such a case, there arises a problem of a bad connection.
In the related art, a height of the solder bumps 102 is determined by a thickness of the mask 108 (hereinafter, the thickness T) and a diameter of the mask openings 110 formed in the mask 108 (hereinafter, the diameter L). That is to say, the solder bumps 102 having a greater height may be formed with a larger thickness T and a larger diameter L.
However, when the thickness T of the mask 108 is too large, there may be a problem in the screen printing process that the mask openings 110 are not sufficiently filled with the solder paste 112.
Also, when the diameter L of the mask openings 110 is too large, the neighboring mask openings 110 cannot be closely situated. Thereby the distance between the neighboring solder bumps 102 becomes large, and a problem arises that the pitch of the solder bumps 102 may not be fine.